Unmanned vehicles are widely used in underwater (also referred to as subsea) operations. Autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) are used for military operations, monitoring marine life, surveying seabed geology and archeology, salvage operations, as well as a multitude of tasks in connection with the extraction of hydrocarbons from subsea wells.
An AUV is generally a pre-programmed robot which is capable of operating under water without any real-time input from an operator.
An ROV is generally an unmanned, highly maneuverable, subsea mobile vehicle, having propulsion devices (e.g. thrusters) and equipment such as torque tools, manipulator arms (including cutters and grippers), cameras (video and stills), lights, sampling devices, etc. An ROV is normally connected to a device often referred to as a tether management system (TMS) via a neutrally buoyant tether. The tether containing electrical conductors and fiber optics for communicating electrical power, video signals, and data signals. The TMS typically comprises a cage holding a reel for storing and paying out tether, and may also comprise means for temporarily securing the ROV.
In use, the TMS is normally suspended (from e.g. a ship's crane) by a load-bearing umbilical cable which also contains power and communication means. The umbilical cable extends between the TMS and a control room aboard a surface ship, from where the ROV is controlled. The TMS is normally suspended in the water during ROV operations, below the splash zone.
Subsea operations in general also rely on extensive use of tools, units and equipment, many of which are remotely operated (e.g. by ROV's). Examples of such operations are BOP intervention, wellhead maintenance, and well intervention.
Such subsea operations, including the operation of the TMS and ROV, require considerable equipment and ancillary systems; for example a Power Distribution Unit (PDU), Hydraulic Power Unit (HPU), Instrumentation Power Assembly (IPA) and a Launch-and-Recovery System (LARS), a control room and various instruments and control interfaces. These systems and equipment are normally installed at the location of operation (e.g. on a vessel, a platform or a drilling rig), often in containers or on skids. This equipment, in addition to the ROV and the TMS, requires extensive logistics and often takes several days or weeks to mobilize to the intended location. It occupies a significant area and (in the case of a floating vessel) allowable variable deck load.
It is therefore a need for an improved system and method of performing these subsea operations.
The prior art includes WO 01/21476 A1, which discloses an AUV which is releasably connected to a flying latch vehicle. The vehicle is connected via a tether to a subsea tether management system (TMS). The TMS is connected via a load-bearing umbilical cable to a topside launch and recovery device.